Exp Brain Res (1994) 100:495-502 9 Springer-Verlag 1994 John J. Jeka 9 James R. Lackner Fingertip contact influences human postural control Received: 10 September 1993 / Accepted: 4 April 1994 Abstract Touch and pressure stimulation of the body surface can strongly influence apparent body orienta- tion, as well as the maintenance of upright posture dur- ing quiet stance. In the present study, we investigated the relationship between postural sway and contact forces at the fingertip while subjects touched a rigid metal bar. Subjects were tested in the tandem Romberg stance with eyes open or closed under three conditions of fingertip contact: no contact, touch contact (< 0.98 N of force), and force contact (as much force as desired). Touch contact was as effective as force contact or sight of the surroundings in reducing postural sway when compared to the no contact, eyes closed condition. Body sway and fingertip forces were essentially in phase with force contact, suggesting that fingertip contact forces are physically counteracting body sway. Time de- lays between body sway and fingertip forces were much larger with light touch contact, suggesting that the fin- gertip is providing information that allows anticipatory innervation of musculature to reduce body sway. The results are related to observations on precision grip as well as the somatosensory, proprioceptive, and motor mechanisms involved in the reduction of body sway. Key words Fingertip 9 Posture control Spatial orientation 9 Touch 9 Human Introduction The primary sensory inputs to postural control are visu- al, proprioceptive, and vestibular (Nashner 1981). Less well studied is the influence of somatosensation on pos- tural equilibrium. In combination with proprioceptive inputs from the legs and ankles, somatosensory stimula- J. J. Jeka ([~) 9 J. R. Lackner Ashton Graybiel Spatial Orientation Laboratory, Brandeis University,Waltham, MA 02254, USA, e-mail: jeka@binah.cc.brandeis.edu tion from contact of the feet with the support surface has been shown to play an important role in maintaining upright stance (Diener et al. 1984). Moreover, a growing body of evidence suggests that touch and pressure cues from any part of the body in contact with a stable exter- nal surface may have a profound influence on apparent body orientation (Lackner 1981, 1992). This implicates a larger role for touch cues in the control of posture than presently conceived. The influence of touch inputs has been illuminated particularly by studies of body orientation illusions. Several studies have shown that somatosensory stimu- lation of the hands or feet can elicit illusions of body motion in blindfolded subjects (Brandt etal. 1977; Lackner and DiZio 1984), even to the point of inducing nystagmus compensatory for the direction of apparent body motion. Moreover, Gurfinkel and Levik (1994) have shown that the cervico-ocular illusion, the sensa- tion of head rotation in space when the trunk is rotated at slow speeds with respect to the stationary head, is suppressed by grasping a rigid ground-based handle during trunk rotation. The tactile and proprioceptive cues from the arm provide veridical information about trunk orientation to a stable external referent, overrid- ing the body-centered reference. Interestingly, when the handle was compliant rather than rigid, suppression of the cervico-ocular illusion was not reported. Tendon vibration studies have also revealed that contact with an external surface can modify propriocep- tive inputs to the perception of body orientation. If the achilles tendons of a standing subject are vibrated while he or she is restrained in position and blindfolded, illu- sory forward body tilting is reported, centered around the ankles. However, if the subject is provided with con- tact cues through a bite plate, the pivot point of tilt often changes from ankles to head. Experienced tilt occurs despite no change in stimulation to the otolith recep- tors, which are often considered the primary influence on the preception of postural upright (Benson 1982; Howard 1986). Furthermore, the patterns of body mo- tion experienced during z-axis recumbent rotation of